A gas turbine unit comprises for example a compressor, a combustion chamber and a turbine. Intake air is compressed in the compressor and then mixed with a fuel. The mixture is burned in the combustion chamber, with the combustion waste gases being fed to the turbine. The turbine extracts thermal energy from the combustion waste gases and converts it to mechanical energy.
During combustion of the mixture thermoacoustic oscillations occur in the combustion chamber and are propagated in a wave. A standing wave can develop in the combustion chamber at a resonant frequency, which damages the combustion chamber in an irreversible manner. These thermoacoustic problems represent a problem during the design and operation of new combustion chambers, combustion chamber parts and burners for such gas turbines.
To reduce thermoacoustic oscillations therefore Helmholtz resonators for example are deployed in the prior art to damp the oscillations. A Helmholtz resonator generally comprises a volume with air contained therein or a different gas. Connected to the volume is a pipe, known as the resonator pipe, in which air or gas is similarly present, and which opens into the combustion chamber. The air or gas in the volume and in the resonator pipe form a spring-mass system, with the air or gas in the volume forming the spring and the air or gas in the resonator pipe forming the mass. When the spring-mass system oscillates at a resonant frequency, which is determined by the volume, the cross-sectional surface of the resonator pipe and the length of the resonator pipe, the Helmholtz resonator behaves in the manner of an opening of infinite length, which makes it impossible for a standing wave to form at resonant frequency.
However a Helmholtz resonator has a fixed resonant frequency. The occurrence of thermoacoustic oscillations can therefore only be effectively suppressed for frequencies, which correspond to the resonant frequency of the Helmholtz resonator or are close to it. The effect of the Helmholtz resonator is significantly reduced for frequencies that differ from it.